In industrial combustion processes, it is a goal to burn out flue gases efficiently and as completely as possible. Efficient flue gas burnout is characterized by low concentrations of incompletely burned products of combustion, such as CO, hydrocarbons, and particulate carbon (soot particles). The corresponding emission limits are usually defined in the relevant legal provisions. In Germany, for example, the limits on carbon monoxide, CO, and hydrocarbons, CnHn are laid down in the 17th Ordinance implementing the Federal Immission Control Act (Bundesimmissionsschutzverordnung, BlmSchV).
Fuels such as household waste, biomass, or coal, which have varying moisture contents, are very inhomogeneous. Because of their highly heterogeneous composition, such fuels have widely varying heating values. Therefore, today, complex combustion control systems including infrared detectors (IR cameras, infrared cameras) are used in combustion chambers of industrial combustion systems. In grate combustion systems, the position of the flames from the bed of solids is detected by the infrared radiation from the fuel bed using an IR camera. The detected wavelength (e.g., 3.9 μm) is in a range in which combustion gases themselves have no emissivity. This information is used to control the kinematics of the grate and/or the various primary gas flows passing through the bed of solids. This makes it possible to achieve an essentially complete burnout of the solid matter in the slag.
When flue gas from non-uniform combustion exits a combustion chamber (for example, from a solid bed burnout zone), such flue gas usually has locally high concentrations of incompletely burned compounds such as CO, hydrocarbons, and soot. The gas flow exiting the combustion bed is characterized by the formation of streaks with enormous local and temporal variations in the concentration of the aforementioned incompletely burned compounds and of the oxygen concentration. These streaks extend through the flue gas burnout zone in the first radiation pass. Homogeneous mixing of the flue gas, and thus complete burnout thereof, is frequently not possible due to insufficient time and insufficient turbulence. Therefore, to remedy incomplete burnout of the flue gases, an oxygen-containing secondary gas is introduced into the flue gas burnout zone. The total amount of this secondary gas is selected such that a defined excess of oxygen (minimum oxygen concentration) is always maintained downstream of the flue gas burnout zone. The minimum oxygen concentration is limited by the minimum combustion temperatures required downstream of the flue gas burnout zone.
DE 103 47 340 A1 describes a device for optimizing the flue gas burnout in combustion systems having a solid bed burnout zone and a flue gas burnout zone. That device includes several controllable nozzles for introducing oxygen-containing secondary gas into an effective area of the flue gas burnout zone. The determination of the various incompletely burned gas components (CO and hydrocarbons) in the effective area is accomplished by measuring the radiation intensity using an infrared camera or any other spectral measurement device. The data acquired in this manner is converted into control signals for each of the controllable nozzles to enable controlled introduction of secondary gas.
However, the device and associated method are used for non-selective determination of incompletely burned gas components in the flue gas. In the process, both incompletely burned gases and solid components (e.g., soot) are determined in the form of a sum signal, with no weighting between the various components being possible. Furthermore, it may happen that areas where, due to the lack of combustion gases, no combustion activity takes place are also identified as being areas of incompletely burned flue gases (cross-sensitivities between the emissivities of CO2 and H2O) In the latter case, injection of an oxygen-containing secondary gas would not cause the gases to be after-burned, but to be diluted and cooled.